The most highly evolved structure in the human brain is the neocortex, which is responsible for the higher cognitive functions unique to humans. Despite its importance, little research has been done to understand how the human neocortex develops, especially during the period when most neurogenesis takes place, the second trimester of gestation. We have recently characterized a unique population of neural stem cells (oRG cells) in the developing human neocortex responsible for the majority of cortical neuron production. These cells reside in a germinal region known as the outer subventricular zone (OSVZ). Notably, the OSVZ is not found in the rodent, implying that using the rodent as a model system to understand human neocortical development has significant limitations. The aim of the proposed research is to further understand the mechanisms by which OSVZ progenitor cells contribute to human neocortical development, focusing on four areas: 1) the origin of oRG cells and their neural stem cell properties; 2) the cellular and molecular mechanisms that maintain oRG cell identity and self-renewal during neurogenesis; 3) the amplifying divisions that oRG cell daughters undergo to increase neuronal production; and 4) how oRG cells are required for the production and correct layering of neuronal subtypes in the neocortex. With innovative approaches such as cell labeling and clonal analysis, real-time imaging of cellular behaviors, and molecular genetic techniques in organotypic slice-cultures, the proposed research aims to provide a solid new foundation for understanding human neocortical development. This framework will be essential to correctly understand human neurological diseases that have genetic or developmental origins, ranging from cortical malformations such as lissencephaly and microcephaly to more subtle defects like epilepsy, autism, and schizophrenia. Knowing the proper developmental sequences by which cortical neurons are generated will also be important for cellular regeneration or transplantation therapies for neurological diseases.